1. Field of the Invention
The present invention relates to a compound microscope of an optical microscope and a scanning probe microscope.
2. Description of the Related Art
A scanning probe microscope (SPM) is a scanning microscope, which mechanically scans a mechanical probe by a scanning mechanism to obtain information on a sample surface, and is a generic name of a scanning tunneling microscope (STM) anatomic force microscope (AFM), a scanning magnetic force microscope (MFM), a scanning near field optical microscope (SNOM), etc. The scanning probe microscope raster scan a sample and the mechanical probe relatively to each other in XY-directions to obtain surface information regarding a desired sample region through the mechanical probe, mapping and displaying the surface information on a TV monitor.
Among others, the AFM is a most widely used device, and comprises, as major mechanical features, a cantilever having a mechanical probe at its free end, an optical displacement sensor to detect the displacement of the cantilever, and a scanning mechanism to scan the mechanical probe and a sample relatively to each other. As the optical displacement sensor, a lever-type optical displacement sensor is most widely used because of its simple configuration and high displacement detection sensitivity. A light beam having a diameter of several μm to several ten μm is applied to the cantilever, and a change of the reflection direction of the reflected light depending on the curve of the lever is detected by, for example, a two-segments photodetector, so that the movement of the mechanical probe at the free end of the cantilever is detected and a corresponding electric signal is output. The scanning mechanism is controlled in a Z-direction so that the output is kept constant, while the scanning mechanism is scanned in the XY-directions at the same time, so that configurations of the sample surface are mapped and displayed on a monitor of a computer.
This AFM is generally combined with an inverted optical microscope in order to observe a bio-sample in a liquid. This is because the observation using the inverted optical microscope is advantageous not only to obtaining information on the sample but also to positioning the cantilever at a particular part of the sample.
When attempting to observe the motion of the bio-sample, it is observation velocity to be required for the AFM. For this purpose, one screen should be obtained within one second, preferably within 0.1 seconds. For higher velocity of the AFM, devices that are currently available on the market have reached a level that can achieve the goal regarding the periphery of electric circuits of the AFM device, while problems lie in the mechanical features. Particularly, these mechanical features include the scanning mechanism having a high scanning velocity, the cantilever that is flexible and has a high resonant frequency, and the optical-lever-type optical displacement sensor capable of detecting the displacement of the cantilever. For example, when an image having 100 pixels in the X-direction and 100 pixels in the Y-direction is loaded within 0.1 seconds, the scanning mechanism is required to attain a scanning frequency of 1 kHz or more in the X-direction, a scanning frequency of 10 Hz or more in the Y-direction, and a scanning frequency of 100 kHz or more in the Z-direction.
The high-frequency cantilever suited to the observation of the bio-sample preferably has a spring constant of 1 N/m or less, and a resonant frequency of 300 kHz. This cantilever has extremely small dimensions that are about one tenth of the dimensions of cantilevers that are currently available on the market. For example, a cantilever made of silicon nitride has a length of 10 μm, a width of 2 μm, and a thickness of 0.1 μm. This cantilever has a spring constant of approximately 1 N/m, a resonant frequency in the atmosphere of approximately 1.2 MHz, and a resonant frequency in the liquid of approximately 400 kHz.
The optical displacement sensor further requires a light focusing optical system to form a spot of converged light having a diameter of several μm or less in order to detect an extremely small displacement of the cantilever.
As described above, for the high-velocity observation of the bio-sample by the AFM, it is preferable that a small cantilever flexible and having a high resonant frequency is available and that a scanning mechanism to perform high-velocity scanning is provided.
Jpn. Pat. Appln. KOKAI Publication No. 2002-82036 has disclosed a compound microscope in which an inverted optical microscope is combined with an AFM to meet the needs.